Tire non-uniformity relates to the symmetry (or lack of symmetry) relative to the tire's axis of rotation in certain quantifiable characteristics of a tire. Conventional tire building methods unfortunately have many opportunities for producing non-uniformities in tires. During rotation of the tires, non-uniformities present in the tire structure produce periodically-varying forces at the wheel axis. Tire non-uniformities are important when these force variations are transmitted as noticeable vibrations to the vehicle and vehicle occupants. These forces are transmitted through the suspension of the vehicle and may be felt in the seats and steering wheel of the vehicle or transmitted as noise in the passenger compartment. The amount of vibration transmitted to the vehicle occupants has been categorized as the “ride comfort” or “comfort” of the tires.
Tire uniformity characteristics, or attributes, are generally categorized as dimensional or geometric variations (radial run out (RRO) and lateral run out (LRO)), mass variance, and rolling force variations (radial force variation, lateral force variation and tangential force variation, sometimes also called longitudinal or fore and aft force variation). Uniformity measurement machines often measure the above and other uniformity characteristics by measuring force at a number of points around a tire as the tire is rotated about its axis.
Once tire uniformity characteristics are identified, correction procedures may be able to account for some of the uniformities by adjustments to the manufacturing process. Some of the uniformities may be hard to correct during the manufacturing process and so additional correction procedures are needed to correct remaining non-uniformities of cured tires. A number of different techniques may be available, including but not limited to the addition and/or removal of material to a cured tire and/or deformation of a cured tire.
One known technique for correcting tire non-uniformities is the use of laser ablation along a bead portion of the tire. For instance, WO 2011/002596, which is incorporated by reference herein for all purposes, discloses the use of laser ablation along various tracks on the bead portion of a tire, such as along the bead seat portion, the lower flange portion, and the upper flange portion of the tire. In particular, an ablation pattern for the tire beads is calculated to reduce the magnitude of one or more harmonics of at least one uniformity parameter. Material along the bead portion of the tire is then selectively removed using the calculated laser ablation pattern.
Known ablation techniques typically use either a variable speed approach or a variable power approach to selectively remove material according to the calculated ablation pattern. In a variable speed approach, the ablation device is operated at a fixed power level while selectively rotating the tire with variable rotational speed to achieve the desired ablation pattern. In a variable power approach, the tire is rotated at a fixed speed and the ablation device is powered at varied levels to accomplish the desired ablation pattern. Combined variations of both power and speed can also be used.
These ablation techniques can typically only ablate a single track along the bead portion of the tire during a single pass of the ablation device. Multiple passes of the ablation device around the tire can be required to ablate multiple tracks along, for instance, two or more of the bead seat portion, the lower flange portion, and the upper flange portion, leading to increased processing time. Use of lower power ablation in variable power ablation processes can lead to even further increased processing times to achieve a desired ablation pattern. It can also be difficult to simultaneously implement multiple objective ablation patterns, for instance to correct both radial force and lateral force parameters, in a single pass of the ablation device using variable power or variable speed ablation techniques.
Thus, a need exists for an ablation technique that reduces the time necessary to achieve a desired ablation pattern in the bead portion of a tire. A technique that can address multiple uniformity parameters and ablate multiple tracks along the bead portion of a tire during a single pass would be particularly useful. A technique that can allow the independent ablation of both beads of the tire at the same time would also be particularly useful.